In the data communications field, a packet is a finite-length (generally several tens to several thousands of octets) digital transmission unit comprising one or more header fields and a data field. The data field may contain virtually any type of digital data. The header fields convey information (in different formats depending on the type of header and options) related to delivery and interpretation of the packet contents. This information may, e.g., identify the packet's source or destination, identify the protocol to be used to interpret the packet, identify the packet's place in a sequence of packets, provide an error correction checksum, or aid packet flow control.
Typically, packet headers and their functions are arranged in an orderly fashion according to the open-systems interconnection (OSI) reference model. This model partitions packet communications functions into layers, each layer performing specific functions in a manner that can be largely independent of the functions of the other layers. As such, each layer can prepend its own header to a packet, and regard all higher-layer headers as merely part of the data to be transmitted. Layer 1, the physical layer, is concerned with transmission of a bit stream over a physical link. Layer 2, the data link layer, provides mechanisms for the transfer of frames of data across a single physical link, typically using a link-layer header on each frame. Layer 3, the network layer, provides network-wide packet delivery and switching functionality—the well-known Internet Protocol (IP) is a layer 3 protocol. Layer 4, the transport layer, can provide mechanisms for end-to-end delivery of packets, such as end-to-end packet sequencing, flow control, and error recovery—Transmission Control Protocol (TCP), a reliable layer 4 protocol that ensures in-order delivery of an octet stream, and User Datagram Protocol, a simpler layer 4 protocol with no guaranteed delivery, are well-known examples of layer 4 implementations. Layer 5 (the session layer), Layer 6 (the presentation layer), and Layer 7 (the application layer) perform higher-level functions such as communication session management, data formatting, data encryption, and data compression.
Packet-switched networks provide an efficient switching mechanism for the delivery of packetized data traffic. The “Internet” is a collection of interconnected packet-switched networks that use layer 3 Internet Protocol (IP) as a packet delivery mechanism. Each packet-switched data network typically contains a core or backbone, made up of switches and routers connected by high-speed layer 1/layer 2 links (as used herein, a router is a device that performs packet-by-packet forwarding based on packet header fields above layer 2, whereas a switch is a layer 2 forwarding or bridging device).
In the Internet model, the core network has no centralized control entity governing how each packet will traverse the network. Instead, each router is highly optimized for the task of forwarding packets across the network, and maintains dynamic routing tables that allow it to make packet forwarding decisions autonomously (although routing information is shared between routers).
Historically, much of the traffic on the Internet has consisted of traffic between large computer hosts, each host connecting networked computer users at a government, educational, or commercial institution to the Internet. Today, however, a significant portion of network traffic goes through an Internet Service Provider (ISP) or other similar gateway. An ISP provides Internet access to residential customers, small businesses, and other organizations, typically via the PSTN (Public Switched Telephone Network). ISP customers connect their computing devices to their ISP using, e.g., an analog modem and a standard POTS (Plain Old Telephone Service) connection, a wireless phone connection, an ISDN (Integrated Services Digital Network) connection, a Digital Subscriber Line (DSL), or a cable modem.
Many ISPs also offer additional network access capabilities, such as Virtual Private Networking (VPN), using protocols such as L2TP (Layer 2 Tunneling Protocol). Without L2TP, a remote user could dial in to a private data network by initiating a PSTN physical connection to a network access server (NAS) on that private network. A Point-to-Point Protocol (PPP) layer 2 link established across this connection would then allow the user to communicate with the NAS. L2TP removes the requirement that the user dial in to the private network directly, by allowing the layer 2 endpoint and the PPP endpoint to reside on different devices connected to a packet-switched network. With L2TP, the user dials in to an ISP, for example. The ISP sets up a packet tunnel to a home gateway (HGW) in the private network, and PPP frames are tunneled from the ISP to the HGW in IP packets. Thus L2TP, and similar protocols, allow private networks to be extended to virtually any location connected to the Internet.
Finally, an ISP (or private NAS) can also offer voice-over-packet network (e.g., VoIP) services. With VoIP, a voice data stream is packetized and transmitted over the packet network. If the calling party or the called party (or both) do not have a “soft(ware) phone” or an IP phone, a call's bearer channel data will require translation, e.g., by an ISP, between the digital time-division-multiplexed (TDM) pulse-code-modulated (PCM) format used by the PSTN and the packet format used by the packet network. When the ISP supplies translation, the ISP will typically implement sophisticated algorithms such as voice activity detection, echo cancellation, compression, and buffering in addition to packetization, in order to reduce call bandwidth while maintaining an acceptable quality of service.
Whether users wish to simply connect their computers to the Internet, tunnel through the Internet to reach a private network, or transmit voice calls across the Internet, the ISP's high-level technical goal remains the same: to serve as a packet network traffic aggregation point for a large number of users with relatively low-speed data connections. As demand increases for network access, virtual private networking, and packet voice, ISPs continue to search for cost-effective ways to provide these services to more users.
Most ISPs deliver the services described above using a network access server. These devices are a type of router that is specifically designed for the task of routing traffic between a large number of low-speed interfaces (called ingress interfaces) and a small number of high-speed interfaces (called egress interfaces). Such servers, like other routers, use a packet forwarding engine to process and route incoming packets to an appropriate outgoing interface. But in addition to packet forwarding, access servers perform a variety of other specialized, data processing-intensive tasks that are not typically found in other types of routers. These functions are, e.g., those required to support PSTN signaling and bearer channel formats, deliver dial-in PPP endpoint and modem functionality, private network tunneling endpoint functionality, and VoIP-to-PCM conversion. As a result, access servers typically use a high-speed forwarding engine for packet processing and routing, and multiple digital signal processors (DSPs) to provide modem and voice packetization services on the ingress ports.